Interferometry is a useful technique for measuring one or more spatial characteristics of a sample, such as a semiconductor wafer or any other semiconductor or non-semiconductor substrate. Semiconductor wafers are continually being fabricated with tighter specifications. Interferometry systems must, therefore, perform at higher levels of accuracy and precision to meet the needs imposed by modern semiconductor technologies.
One or more detectors of an interferometry system must be focused correctly prior to making measurements in order to achieve matching among different systems. A detector may be adjusted to a focus position by manually focusing on a sharp edge of an artifact disposed proximate to a test surface of a sample. However, the foregoing method does not provide adequate focus for some applications and requires specially designed samples, such as specially patterned wafers, which are often expensive.